The present disclosure relates generally to electrostatic reproduction machines, and more particularly to an apparatus to assist in the transport of sheets through such machines.
In a typical toner image reproduction machine, for example an electrostatic printing process machine, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced. Exposure of the charged photoconductive member selectively dissipates the charges thereon in the irradiated areas to record an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document.
After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet.
The foregoing generally describes a typical black and white electrostatic printing machine. With the advent of multicolor electrophotography, it is desirable to use an architecture which comprises a plurality of image forming stations. One example of the plural image forming station architecture utilizes an image-on-image (IOI) system in which the photoreceptive member is recharged, reimaged and developed for each color separation. This charging, imaging, developing and recharging, reimaging and developing, all followed by transfer to paper, is done in a single revolution of the photoreceptor in so-called single pass machines, while multi-pass architectures form each color separation with a single charge, image and develop, with separate transfer operations for each color.
In either case, the toner image ordinarily is transferred unfused onto a copy sheet of paper, which is then picked up by a transport mechanism (a pre-fuser transport) for delivery to a fuser assembly where the toner is heated and fused to make a finished copy. Conventional pre-fuser transport mechanisms typically use rotating belts stretched between a drive shaft and an idler shaft with perforations in the belts that allow vacuum pressure from a blower to be drawn through holes in a plate below the belts, and through the belts to the sheet. The vacuum pressure assists each sheet of paper that has an image on it via electrically charged toner particles, to be pulled off the photoreceptor and acquired on the pre-fuser transport, without disturbing the unfused image on the sheet, especially in the transfer zone. The sheet is then transported and delivered to the fuser module where the toner particles are heated and pressure-fused to the sheet.
Referring to FIG. 1, a typical electrostatic or electrophotographic printing machine is depicted. The machine employs a photoconductive belt that is preferably made from a photoconductive material coated on a grounding layer, which, in turn, is coated on an anti-curl backing layer. The photoconductive material is made from a transport layer coated on a generator layer. The transport layer transports positive charges from the generator layer. The belt moves in the direction of the arrow to advance successive portions of the photoconductive surface sequentially through the various processing stations disposed about the path of movement thereof. Initially, a portion of the photoconductive surfaces passes through a charging station A. At the charging station A, two corona generating devices charge photoconductive belt to a relatively high, substantially uniform potential.
Next, the charged portion of photoconductive belt is advanced through an imaging station B. At the imaging station B, a document handling unit H is positioned over a platen of the printing machine. The document handling unit H sequentially feeds documents from a stack of documents to be copied. After imaging, the original document is returned to a document tray by either of two paths. If a simplex copy is being made or if this is the first pass of a duplex copy, the original document is returned to the document tray via a simplex path. If this is the inversion pass of a duplex copy, then the original document is returned to the document tray through the duplex path.
Imaging of the document is typically achieved by two Xenon flash lamps which illuminate the document on the platen. Light rays reflected from the document are transmitted through a lens that focuses light images of the original document onto the charged portion of the photoconductive surface of belt to selectively dissipate the charge thereon. This event records an electrostatic latent image on photoconductive belt which corresponds to the informational areas contained within the original document. Thereafter, the photoconductive belt advances the electrostatic latent image recorded thereon to the development station C.
At the development station C, a magnetic brush developer unit delivers developer to developer rolls. The photoconductor belt is partially wrapped about the rolls to form extended development zones. Thus, the developer rolls advance developer material into contact with the electrostatic latent image. The latent image attracts toner particles from the carrier granules of the developer material to form a toner powder image on the photoconductive surface of belt. The belt then advances the toner powder image to a transfer station D.
At the transfer station D, a copy sheet is moved into contact with the toner powder image. First, the photoconductive belt is exposed to a pre-transfer light from a lamp (not shown) to reduce the attraction between the belt and the toner powder image. Next, a corona generating device charges the copy sheet to the proper magnitude and polarity so that the copy sheet is tacked to the photoconductive belt and the toner powder image attracted from the photoconductive belt to the copy sheet. After transfer, the corona generator charges the copy sheet to the opposite polarity to detach the copy sheet from the belt. A conveyer typically advances the copy sheet to a fusing station E.
The fusing station E includes a fuser assembly which permanently affixes the transferred toner powder image to the copy sheet. Preferably, the fuser assembly includes a heated fuser roller and a pressure roller with the powder image on the copy sheet contacting fuser roller. The fuser roll is internally heated by a quartz lamp. Release agent, stored in a reservoir, is pumped to a metering roll. A trim blade trims off the excess release agent. The release agent transfers to a donor roll and then to the fuser roll.
After fusing, the copy sheets may be fed through a decurler 55. The decurler 55 bends the copy sheet in such a way that the sheet curl produced during the fusing operation is substantially reduced. The decurler may be configured as disclosed in U.S. Pat. No. 6,314,268, issued to the assignee of the present invention on Nov. 6, 2001. The disclosure of this application, and particularly the description of the reproduction machine and the interface between the fusing station and the decurler, is incorporated herein by reference.
Forwarding rollers 62 then advance the sheet along a transport chute 60 to a duplex solenoid gate 65. This gate guides the sheet to a finishing station F or to a duplex tray 70. The duplex tray provides an intermediate storage for those sheets that have been printed on one side and on which an image will be subsequently printed on the second, opposed side thereof—i.e., the sheets being duplexed. In order to complete duplex copying, the simplex sheets in tray 70 are fed back to the transfer station D for transfer of the toner powder image to the opposed sides of the copy sheets. The duplex sheet is then fed through the same path as the simplex sheet to be advanced to the finishing station F.
Copy sheets are fed to the transfer station D from a high speed feeder tray I or from secondary trays 72. In many machines, a vacuum feed belt feeds successive uppermost sheets from the trays to the transfer station D.
One problem with electrostatic reproduction machines of the type just described is that the copy sheets are often difficult to extract from the fuser rolls. Many machines incorporate a stripper apparatus that separates the leading edge of a sheet from a heated fuser roll. One example of an apparatus of this type is disclosed in U.S. Pat. No. 6,782,228, issued to the assignee of the present invention on Aug. 24, 2004.
Another problem with electrostatic reproduction machines is that the copy sheets leaving the fusing station E are at a high enough temperature to cause blocking—i.e., successive sheets stick together. Moreover, in the duplexing mode, the high sheet temperature often causes defects in the image transferred in the duplex pass through the machine.
One solution to this problem has been to lengthen the transport path of the sheet as it exits the fusing station. This approach inherently increases the space required between the fusing station E and the solenoid gate 65. In addition, the lengthened transport path usually requires the use of nip rollers, such as the rollers 62 shown in FIG. 1, which can mar the transferred image. Another approach to solving the high temperature problem has been to direct machine air flow across the sheet as it exits the fusing station. In some cases, this air flow may be disruptive to the smooth transport of the sheet to the subsequent stations. Moreover, the amount of air flow required to adequately cool sheets leaving the fusing station at high page-per-minute rates typically requires an air flow system that is size or cost prohibitive.
There is a need for a transport apparatus that addresses the high temperature problem associated with sheets discharged from a typical fusing station. This need is particularly acute for high volume reproduction machines and for machines with duplex capabilities.